The correct answer is: D. All of the above.
In Ward-Leonard control, the speed of the motor is controlled by varying the voltage applied to the motor. The voltage is applied from a DC generator, which is driven by a variable-speed AC motor. The lower limit of speed is imposed by the residual magnetism of the generator, the core losses of the motor, and the mechanical losses of the motor and generator together.
Residual magnetism is the magnetism that remains in a ferromagnetic material after it has been magnetized. In a generator, residual magnetism is what allows the generator to produce a voltage even when it is not being driven by an AC motor. The amount of residual magnetism in a generator depends on the material of the generator’s field poles and the strength of the previous magnetization.
Core losses are the losses that occur in the magnetic core of a motor or generator. These losses are due to eddy currents and hysteresis. Eddy currents are currents that circulate in the magnetic core due to the changing magnetic field. Hysteresis is the loss of energy due to the magnetization and demagnetization of the magnetic core.
Mechanical losses are the losses that occur in the bearings, brushes, and other moving parts of a motor or generator. These losses are due to friction and wear.
The lower limit of speed in Ward-Leonard control is the minimum speed at which the motor can operate. This speed is determined by the residual magnetism of the generator, the core losses of the motor, and the mechanical losses of the motor and generator together. If the voltage applied to the motor is too low, the motor will not be able to produce enough torque to overcome the mechanical losses and the motor will stall.